1. Field of the Invention
The invention relates to a hydraulic combined service brake/park and hold brake system having an accumulator and a booster that automatically boosts the energy storage capacity of the accumulator during the first brake braking cycle(s) to compensate, e.g., for volumetric changes in the system that might take place during prolonged periods of non-use under dropping-temperature conditions. The invention additionally relates to a method of using such a system.
2. Discussion of the Related Art
Many vehicles employ a parking brake system or arrangements operative to maintain the service brake(s) of the system in an engaged condition during parking. These “park and hold brake” systems frequently employ a mechanism associated with a primary brake pedal to selectively latch the brake pedal in a locked position in which its brakes are engaged, while enabling brake pedal release when the operator wishes to effect further movement of the vehicle. See, for example, U.S. Pat. Nos. 1,927,209, 1,985,319, 2,551,743, 2,816,333, 4,036,078, 4,218,936, 4,310,064, and 4,867,829. The brakes of most of these systems are operated by depressing the brake pedal and are locked and/or unlocked by operating a secondary pedal or pad located on or near the primary brake pedal. See, for example, U.S. Pat. Nos. 4,306,078 and 4,867,289.
In the case of relatively small utility and recreational vehicles which undergo frequent stopping, such as golf cars and the like, it is particularly important from a safety standpoint to be capable of holding the vehicle in a stopped position when on an upwardly or downwardly inclined grade. For example, safety regulations require that certain vehicles, such as golf cars, be capable of maintaining a braked condition on a 30% grade under full load for a substantial period of time. Most of these small vehicles employ mechanical, cable-actuated brake systems for both service braking and park and hold braking. While these mechanical systems are acceptable for many applications, they exhibit marked disadvantages when compared to hydraulic brake systems of the type employed by most heavy-duty vehicles. For instance, due at least in part to the fact that mechanical brake systems exhibit substantial hysterisis when compared to hydraulic systems, the operator of the typical mechanical brake system must impose a substantial force to the primary brake pedal to effect braking of the typical mechanical brake system, and a similar force is required to actuate an associated brake locking arrangement. These mechanical brake systems also require a relatively large force to release the brake pedal from its locked position. A further disadvantage resulting from the relatively large operating forces required to actuate and/or release mechanical brake systems is the need for high strength structural components to withstand continued service without associated maintenance problems. The key advantage to reducing the brake system hysterisis is braking control. A lower hysterisis system can have its braking torque increased or reduced easily. A higher hysterisis system feels “sticky.”
The park and hold brakes of some small vehicles can be released by selective actuation of either an auxiliary lever or pedal located on or near the brake pedal or by the accelerator pedal. However, brake release in systems of this type typically requires that the accelerator pedal must be depressed through a substantial stroke and/or against substantial resistance to accelerator pedal movement. As a result, the operator must impart so much force to release the brakes that the accelerator pedal is driven through a substantial percentage of its service stroke before the brakes are released, resulting in near-instantaneous and relatively rapid vehicle acceleration upon brake release. The vehicle therefore jerks forward at substantial operator discomfort and at the risk of loss of vehicle control.
The brake pedals of many park and hold brake systems are locked by a dog and detent mechanism that has one or more latching points and that therefore can permit the vehicle's brakes to be locked in a number of progressively-more heavily braked settings. At least the lightest settings of some of these multipoint latching systems might not produce a strong enough holding force to assure adequate braking on steep slopes. This problem can be avoided through the use of a single latching point system that latches the brake pedal at or beyond a location at which the vehicle's brakes lock the wheels from rotation. However, single latching point systems usually exhibit a pronounced snapback effect and high efforts to set on the brake pedal (produced by the rapid release of accumulated energy from a fully-engaged brake) that produces an undesirable, relatively loud noise and system hammering and that produces substantial wear and tear on the brake pedal and related components. In some systems, this snap back can be so severe as to risk operator injury.
Another problem associated with systems having either single point latching or multipoint latching is that the latching components of these systems contact one another at several different points in both the latching and unlatching operations, producing several distinct audible clicks that may confuse the operator into believing that the brakes are locked when they are not and/or that the brakes have been released when they are still locked.
Yet another problem associated with known hydraulic combined service brake/park and hold brake systems is that they lack any structure that ensures pressure retention in the brakes in the event of gradual relatively minor pressure reduction in the system. This gradual pressure reduction, generally is known as “creep,” may occur, e.g., due to fluid seepage into seals and other elastomeric components of the brake system as well as leakage at metal to metal seal points. Absent some mechanism to make up for the holding energy lost due to creep, the brake holding forces may decrease over time to a level that that risks unintended partial brake release and consequent unwanted vehicle movement.
Thus, a need exists for an economical and reliable brake system 1) which uses the same hydraulic brakes for both service braking and park and hold braking, and 2) which is relatively simple to actuate both to latch and unlatch the brake pedal during park and hold.
The need also exists for either a hydraulic or mechanical park and hold brake system that can be released by actuation of an accelerator pedal with minimal effort on the part of the operator, thereby permitting the operator to “feather” accelerator pedal operation and provide smooth, gradual vehicle acceleration.
There is also a need to store some of the energy generated manually upon actuation of a brake pedal of a hydraulically actuated park and hold brake system and to permit that energy to be released as needed to make up for the minor seal creep that may occur over time.
The need also exists to damp brake pedal return following a braking operation so as to reduce wear on brake system components and to reduce or eliminate operator discomfort associated with pedal snapback and to give the system a quality feel.
A problem associated with accumulator-equipped hydraulic park and hold braking systems is that the accumulator of the system may not store sufficient energy to make up for higher magnitude fluid pressure losses resulting, e.g., from severe temperature drops. That is, a well-designed accumulator will store sufficient energy upon brake latching to maintain the brakes in their applied condition despite low magnitude volumetric changes in the system. However, an accumulator may be incapable of storing sufficient energy to compensate for larger pressure changes resulting, e.g., from severe temperature drops. For instance, the brakes of a golf car or similar vehicle may be latched on a relatively warm autumn day, and the vehicle may then be stored through the winter, where it is subject to a temperature drop of 70° F. The braking system may suffer such a severe volumetric change and resultant pressure drop as a result of this temperature drop that the system's accumulator cannot store enough energy to pressurize the system sufficiently to maintain the brakes in their engaged condition. The “creep” that otherwise would be avoided through the incorporation of an accumulator into the system may therefore still present a problem. This pressure loss and resultant strain on the accumulator are especially dramatic in relatively poorly bled systems having relatively large quantities of air in the hydraulic fluid.
An accumulator equipped system could conceivably be designed to compensate for temperature-dependent volumetric-based pressure losses simply by oversizing the accumulator sufficiently to store the additional energy required to compensate for the additional pressure drop resulting from the pronounced temperature drop. However, energizing the oversized accumulator would noticeably increase the effort required to latch the brakes, particularly if the accumulator were manually energized by actuation of the system's brake pedal. Space constraints also prohibit accumulator oversizing in some applications.
These problems could be alleviated by incorporating a manually actuated booster in the system that boosts the energy storage capacity of a brake system's accumulator. However, a manually actuated booster would require the manual manipulation of a slide or similar device to engage the booster. Specifically, when an operator of this type of system desires to park the vehicle for a long term, he or she would have to depress the brake pedal while simultaneously manually actuating the booster, thus blocking the accumulator from returning to its fully released or “home” position. The operator could then release the brake pedal and reapply it to get the advantage of a two-step boost. Once the vehicle incorporating this hypothetical system is parked, the slide or similar device would spring back out of engagement with the accumulator, and the accumulator is free to use all the accumulated energy the boost had afforded. This system would require the operator to be aware of the park function and to use it when appropriate.
The need has therefore arisen to provide a booster that automatically increases the energy storage capacity of an accumulator of a hydraulic park and hold brake system upon normal manipulation of a brake pedal.